STACKITSBM-463 - update docs - rrule,maintenance_window

[aeter]

New rrule and maintenance_window docstrings for serverbackup and serverupdate schedules.

ref STACKITSBM-463

---

Tests:

[~/terraform-provider-stackit] make project-tools
[~/terraform-provider-stackit] make generate-docs
...
[~/terraform-provider-stackit] make fmt
[~/terraform-provider-stackit] golangci-lint run --allow-parallel-runners --timeout=5m --config=./golang-ci.yaml stackit/internal/services/serverbackup/schedule/*.go
[~/terraform-provider-stackit] golangci-lint run --allow-parallel-runners --timeout=5m --config=./golang-ci.yaml stackit/internal/services/serverupdate/schedule/*.go
[~/terraform-provider-stackit] go test stackit/internal/services/serverbackup/schedule/*
ok      command-line-arguments  0.005s
[~/terraform-provider-stackit] go test stackit/internal/services/serverupdate/schedule/*
ok      command-line-arguments  0.004s

Description

relates to STACKITSBM-463

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)